by Alfred Scott
This article appeared in the December 1988 Falco Builders Letter.
Gather 'round me builders, and a story I will tell, of Aerolite and molecules, and alkaline reactions. Of Englishmen, formic acid and urea-formaldehyde. If he had only known about it all, Woody Guthrie would have written a ballad!
We've all gotten so used to using Aerolite glue that it struck me strange that we did not understand the stuff. I became curious about all of this when some builders talked about thinning the glue slightly for laminating. What happens if you thin it? Is it all right to add water when it thickens over time? Why does it thicken like that-is it from evaporation or something else?
Fasten your mental seatbelts, because we're about to depart on a little journey into the world of organic chemistry, and by the time this is over, you'll know more about the urea-formaldehyde reaction than Mrs. Smith knows about her famous pies.
I received my information from Buzz Glade who sent along a page from a book describing the chemical reaction in layman's terms, some interesting historical information from Ciba-Geigy, but by far the most detailed information came from Falco builder Dave Gauger. Dave is a surgeon in Iowa City, Iowa, and by coincidence the Department of Chemistry and Department of Botany at the University of Iowa both share the same building and library.
And it was there that Dave found a book that explained it all: Wood Adhesives, Chemistry and Technology, by Antonio Pizzi and published by Marcel Dekker, Inc, 270 Madison Avenue, New York, NY 10016. Dr. Pizzi is an Italian by birth who lives in South Africa. He has a doctorate in physical chemistry of polymers from the University of Rome, Italy, and a Ph.D. degree in applied organic chemistry from the University of the Orange Free State, South Africa. Dr. Pizzi heads the Wood Chemistry Division of the National Timber Research Institute in Pretoria. It appears that he knows his glues. If you get the book, see pages 60-65, 80-94 and 102.
First, a little refresher in chemistry. You will all remember that atoms have a ring of electrons around them. The number of electrons varies with all kinds of things and we needn't worry about why, but some atoms (like helium) have a complete set of electrons and thus are happy all by themselves. Others don't have a complete ring, and this causes the atom to join with other atoms until the combinations of atoms have a complete set of electrons. The joining together of atoms is called a reaction. Molecules do it, too. When atoms react with each other and become a molecule, they do this by the mechanism of interlocking electron rings. In a compound, the electrons belong to and circle all of the atoms.
The degree to which an atom or molecule is missing a complete ring of electrons is called its valence, and a missing electron is called a "bond", which is a convenient way to keep track of how many of one element will join with another. Hydrogen (H) has one bond, oxygen (O) has 2 bonds, nitrogen (N) has 3 bonds, and carbon (C) has 4 bonds. When you have a complete match of bonds, such as in H2O, the compound is stable and doesn't combine with anything.
What happens when the compounds urea and formaldehyde come together is an extremely complex reaction. The reaction can be catalyzed by an acid or an alkali and heat increases the speed of either reaction. When the acid catalyst is used, the polymer tends to be a longer linear chain with side branches that has good adhesive properties. When catalyzed with an alkaline agent, the polymer is less suitable as an adhesive.
The Aerolite powder contains urea, formaldehyde, a thickening filler (probably cellulose) and probably a few other things only Ciba-Geigy knows about. When you mix the powder with water, all of these things go into solution. They don't combine with the water, but the water lets the molecules swim around and join with each other more easily than if they remained in a dry powder state where the molecules can't move about.
The water never actually enters into the chemical reaction between urea and formaldehyde, in fact, a little water is actually thrown off by the reaction. The acid doesn't enter the reaction either. It's a true catalyst, causing a chemical reaction but not becoming part of it. When Aerolite is cured, the hardened glue is composed of a variety of molecules which may vary from a few hundred to a few thousand atoms. The chemical descriptions of these compounds becomes very complex, but there is a repeatible pattern of links in the molecular chain and all this is well understood. This process of joining their tiny hands makes the substance into a cured resin that binds two pieces of wood together.
Aerolite was originally developed for the deHavilland Mosquito bomber
For more on the Mosquito, check out
Aerolite powder, when mixed with water, is a slightly alkaline solution. The urea and formaldehyde molecules immediately begin to react with each other, but fortunately this process is a slow one. The thickening of Aerolite has little to do with evaporation; in fact, I've mixed some Aerolite and left it in an air-tight plastic container. It eventually hardened to a cheese-like consistency that you could easily break and crumble in your fingers. Just out of curiosity, I poured some acid hardener on this, and it hardened like any other Aerolite-but of course it was worthless as a glue.
Dr. Pizzi devotes many pages to the discussion of the control of the size of the molecules in urea-formaldehyde glue. The chemists working on these compounds operate in the world of molecular riot control, and they have no absolute control to force this molecule to join with that one. The size of the molecules has a lot to do with the properties of the glue. When freshly mixed, the molecules are very small, and the mixture will flow freely into the cracks and crevices of the wood surface. When the molecules are so long that the mixture is cheese-like, it will not flow at all.
In its pure form, urea compounds are thin and watery, and they tend to soak into the wood or run off-in short, they are terrible glues. A thickening agent is added to prevent glue starvation. Almost all glues have some sort of thickening agent, or "filler". Starch, cellulose, wood, wheat and corn flour, ground pecan shells are commonly used by glue manufacturers. Cellulose fillers are not only lighter in color than wood flour, but because of their fibrous nature, they impart to the resin higher mechanical strength and a surprising translucency, which differentiate them very clearly from wood-flour-filled resins.
This certainly sounds like a description of Aerolite to me, for Weldwood Plastic Resin glue (also urea-formaldehyde) is dark brown in color and does not adhere well to birch plywood. My guess is that the size and type of the filler plays a critical role in the performance of the glue. I'd guess that the Aerolite filler is a smaller fiber of strong cellulose while Weldwood uses larger particles of ground walnut shells.
There are many different acids that can be used to catalyze the urea-formaldehyde reaction. In the wood products industry, ammonium sulfate and ammonium chloride at two of the most widely used curing agents. Some are added just as the glue is mixed and spread on the wood. In Weldwood Plastic Resin glue, the catalyst is a compound that is a dry powder that doesn't become an acid until it is mixed with water. This makes the glue simple to use, but you should also be careful to keep the can tightly closed since the humidity in the air can cause the glue to harden.
Interestingly, Dr. Pizzi makes no mention of formic acid at all. It appears that Ciba-Geigy is alone in using that acid, but it's not hard to see why. Formaldehyde decomposes into formic acid by oxidation in the same process that causes alcohol in wine to turn to vinegar. Just by virtue of making formaldehyde, Ciba-Geigy is going to get a lot of formic acid anyway, so why not use it?
Dave Gauger says, "The only part of the formaldehyde/formic acid relationship that has stuck with me over the years as a physician is the fact that methanol is oxidized by enzymes in the liver to formaldehyde which is then oxidized further to formic acid. This is why you go blind if you drink wood alcohol." Formic acid is also one of the "active ingredients" of many bee stings.
The company, in their quaint British way, named the hardeners with the "GB" prefix, for "gap bridging". Ciba-Geigy touts the development of the 'gap bridging' hardeners as a major breakthrough. I found myself wondering if this was just marketing hype from an era that believed Standard Oil of New Jersey when it claimed that Esso gasoline was quite different from Texaco's brew. Truth in advertising has brought many changes in our understanding of the products we use.
And yet, when I read Dr. Pizzi's book, I find him discussing the advantages of absorbant fillers to decrease shrinkage, improve gap filling and decrease cracking of the cured resins. There is so much magic in chemistry, that it's completely possible that the acid hardener does contain something that adds to the gap bridging nature of the resin. I'm happy to accept that the glue itself is gap-filling, and I don't care if the magic potion is in the glue or the hardener. We should all leave the manufacture of the glue to Ciba-Geigy and confine our discussion to the understanding of the glue.
In their instructions for the use of Aerolite (which we publish as Appendix D in our construction manual), Ciba-Geigy says that the recommended proportions of 1 part water to 2 parts powder by weight are approximate values and on vertical surfaces and for filling gaps, less water is preferable. For this they suggest 45 parts of water to 100 parts of powder by weight.
It is very clear that the proportions of water to powder are a result of experimentation by Ciba-Geigy to get a consistency for the glue that gives the best performance and that it is not related to any mechanics of the chemical reaction. I asked Ciba-Geigy about increasing the amount of water in the glue. They replied, "You are correct in your assumption that increased water content reduces the strength of a glue line. This is not only by dilution of the glue but also because a reduced viscosity will result in greater absorption of glue into the wood and away from the glue line. This is especially so with more absorbent timbers. Water content should be kept to a minimum and be increased only to adjust viscosity depending on the application. The consequence of extra water addition will be a reduction in glue line durability or resilience, and we would not recommend such a practice in your industry."
First, I'd like to express my appreciation to Ciba-Geigy for answering my questions, for in this crazy litigious country a lawsuit can hang on every word of advice from a company-it's a wonder they didn't pull their product off the market after hearing from an aircraft company in the U.S.! We should all realize that such answers may be overly conservative for just such a reason. You be the judge and don't try anything different without first running a complete series of tests.
Let's start with some hard numbers. ANC-18 Design of Wood Aircraft Structures lists the shear strength parallel to the grain at 15% moisture content as 720 psi for Sitka spruce, 1,630 psi for birch, and 1,830 for sugar maple. We all use maple for test blocks because it is so much stronger than spruce and if the glue passes with maple, you have an ample margin of safety for spruce. If you think about it, except for scarf joints in plywood, almost every joint in the Falco has spruce on one side of the joint. And since spruce is the weaker of the two woods, you need to achieve adhesion to both pieces and a glue line stronger than spruce.
Opinions vary, but I understand that Aerolite has a shear strength of about 2,500 psi, but this is difficult to test since few woods are that strong. In any event, it's safe to say that a properly made Aerolite joint is probably three times stronger than spruce. There is a considerable margin of safety in this system.
The deHavilland DH-88 racer also used Aerolite.
In the sixties, Jean Peters of Western Aircraft built about seventy Cavalier wing spars using Aerolite. That spar is very similar to the Falco spar, except that the laminations are made of 6mm spruce. To make the glue more acceptable for laminating, Jean thinned the glue to a corn-syrup consistency. His tests with spruce blocks showed the glue joint was of adequate strength. Just out of curiosity, Craig Bransfield tried thinning some Aerolite and used some spruce test blocks. He said the glue performed well.
If you do any experimentation with thinning Aerolite, be sure to use both maple and spruce test blocks-maple to test the strength of the glue and spruce to insure that the porous wood has not destroyed the strength. Most of you should never deviate from the prescribed methods. You should have a very good reason before you do that.
Now to the question of what to do with a thickened resin, Dr. Pizzi says that it is safe to add water to resins with decreased flow, but unacceptable to do this with resins that have totally lost their flow. Dr. Pizzi gives no guidelines as to how much water is too much or how thick the resin can get before you can no longer safely thin it out. You will have to be your own judge of this and run some tests before you actually use such glue on your airplane.
Dave Gauger took some thickened Aerolite that was still pourable and diluted it back to the thickness of freshly mixed Aerolite, and it worked just fine on some maple text blocks. Even so he said, "I was unsure whether or not this was a safe practice so I discarded the stuff. Since Aerolite is the cheapest component in the Falco, I don't feel too bad about dumping out the glue that is too thick to pour. In terms of time expenditure, it is probably just as efficient to mix the stuff in small batches as you go, then to make one big batch and repeatedly try to resuscitate it."
Ciba-Geigy is very cautious about adding water. They say that the Aerolite powder mixed with water "will tend to thicken even without hardener addition. The reaction causing this (assuming no evaporation) is however the same and any thickened Aerolite should really be discarded as having started to gel. Addition of water is not a remedy."
The urea-formaldehyde reaction is a two-stage process. The first is the alkaline condensation to form mono-, di- and trimethylolureas. The formation of these methylolureas is what goes on when you let the glue sit in the bottle. This process can be slowed by putting the bottle in a refrigerator and most Falco builders keep their glue this way for about two weeks before discarding it.
The second stage is the acid condensation of the methylolureas, first to soluble and then to insoluble cross-linked resins. The soluble resins are formed during the stage when the glue becomes rubbery. Excess glue can be scraped or wiped off with a wet cloth. In the acid condensation, the products precipitated from aqueous solutions of urea and formaldehyde, or from methylolureas, are low molecular weight methyleneureas. Thus, the end product is the same in either case and the real question is whether the thickened Aerolite is still an acceptable glue that will flow onto the surface of the wood.
Ciba-Geigy is extremely cautious about keeping mixed Aerolite. "We would recommend that Aerolite 306 be mixed at regular (1-2 daily) intervals to ensure fresh resin is used."
When the Mosquito bomber was built, the Aerolite curing was accelerated by heating. The method used was electrical resistance strip heaters where low voltage, high current electricity was passed through metal plates embedded in the clamping fixtures. This is similar to the common practice of using an electric arc welder to unfreeze buried pipes.
Ciba-Geigy advises "The hardening of Aerolite can be speeded up by raising the temperature of the glue line. However, the equipment for doing this is unlikely to be cost-effective for the large range of different joint sizes you have. There is also the additional problem of distortion due to the non-uniform drying action of this heating, with serious consequences for accurate engineering."
Always check a new batch of glue before using it on your airplane. In all the time we have been dealing with the Falco, I have only heard of one incidence that seemed to be a bad batch of Aerolite. Richard Clements tried Aerolite and said it performed poorly and that the test blocks failed in the glue joint. He gave the batch to a Falco builder friend, Bob Cordray. Bob found the same thing, but he ordered a new batch, and it passed the same tests.
I always encourage builders to read the facts about all of the glues, then get some glue and try it. The simple fact is that there is no one perfect glue for an airplane and certainly not for every builder. But Aerolite has proved to be the glue most favored by Falco builders.
An interesting case is Gary Smith, who had built a Pitts using T-88 and recently restored an Aeronca C3 using T-88. Gary is a very experienced woodworker who has a large custom cabinet making shop just outside of Atlanta. He has one of every piece of woodworking equipment, thirty employees, and he took one look at the wood kits and bought them all. Anyway, Gary had never heard about the poor high-temperature performance of T-88 so he popped some test blocks in the oven. He didn't like what he saw so he got some Aerolite and ran some tests. That's all he uses now, and he says he is surprised to find himself using the glue.
Aerolite is the product of a romance with aviation that began fifty years ago. In 1931, Norman de Bruyne, a professor at Cambridge University founded the Cambridge Aircraft Construction Company in a workshop at the flying school at the Cambridge airport. He was the first pupil of the school, learned to fly and soon bought a Gypsy Moth biplane. Convinced that British aircraft design had 'got stuck in a rut', he and a friend designed and built a low-wing monoplane called the Snark. The project took three years, and he changed the name of the company to Aero Research, and moved it to Duxford, ten miles south of Cambridge and the site of the Ciba-Geigy plant that today makes Aerolite.
The Snark was an all-wood design assembled with casein glue which was the only glue available with even the slightest water resistance, and the plywood was assembled with blood. Dependance on biplanes was one thing, but flying on blood was another, so they set about the search for a better glue. Dr. de Bruyne had already struck up a relationship with Geoffrey de Havilland to act as a consultant with particular reference to the use of plastics in aircraft.
In 1937, Dr. R. E. D. Clark, a chemistry professor from Cambridge, produced an experimental urea-formaldehyde resin for evaluation. A pilot plant was built with a second-hand laundry boiler producing the steam necessary to make the product they called "Aerolite". Later that year Claude Rayner joined Aero Research and took charge of the project, and it was his discovery of GB or 'gap bridging' hardeners, incorporating formic acid, that made Aerolite a practical assembly glue.
A replica of the deHavilland DH-88 Comet.
Dr. de Bruyne continued to work with other "plastics". A laminate of flax roving and paper soaked with liquid phenolic resin and cured under pressure was called Gordon Aerolite. This is the predecessor of today's phenolic sheet and was named for a friend called Gordon, whose family was in the linen business and who supplied de Bruyne with flax after he had been rejected by the glassfibre manufacturers since they did not see "any prospect for glass 'silk' being suitable for molded plastics" and did not want to be associated with a failure.
The tiny Aero Research company continued to struggle, but the glue started to sell. They repaired an delaminating 1929 Desoutter monoplane and made Miles Magister tailplanes for the Air Ministry. During this time, de Havilland's chief engineer began spending time at Aero Research discussing the concept of wood sandwich construction with balsa core. This eventually led to the production of the Mosquito bomber.
As war broke out, the tiny company began to grow and developed the strip heating process to speed the assembly of wood parts. Morris Motors used Aerolite and strip heating to assembly Horsa gliders, as did de Havilland on the Mosquito as well as on other aircraft and in naval launchs and patrol boats.
Other adhesives were developed, Redux (for REsearch at DUXford-get it?) was developed to box aluminum sheet to a balsa core. Fomvar was an early film adhesive. Aerodux was a resorcinol which to this day remains one of the company's most popular glues.
At the end of the war, the company's first efforts to market their products was to the Finnish Plywood Association. The marketing manager met the four-man committee who asked several questions, conferred with each other for a few minutes and then produced an order of 100 tons of Aerolite. The stunned salesman, terrified that they might change their minds, fled from the room.
This large order was a turning point for Aero Research, which spent the next five years working on a plan and financing for truly large-scale low-cost production of urea-formaldehyde resins. In the end, Aero Research was taken over by the Swiss Ciba company, a large multinational group of chemical companies that wanted to expand into England. Ciba had already invented the melamine resins and had produced the Araldite epoxy resins.
Today Ciba-Geigy is one of the largest producers of adhesives in the world. They are a major supplier of epoxy resins, which ends up in strange places-half of their epoxy sales are to paint companies, who use it to seal the inside of paint cans. They now ship their Aerolite resins in 100-ton-capacity tank cars. Most of their Aerolite urea-formaldehyde is shipped to the chipboard industry and those resins are supplied in a completely different form from the glue we use. The Aerolite 306 that we use is a specialty product with such a small market that Ciba-Geigy doesn't even have a U.S. distributor.
All this because a Cambridge Don decided British aviation was stuck in a rut, set out to reform things by building a plywood airplane, and ended up making goop. It sure beats blood.